doc: Change documentation for =0 for non-boolean options

PR bitcoin#12713 changed the interpretation for negation of non-boolean options
(e.g. -noconnect) to no longer set the option to 0, but to remove it
from the options.

I think this is better because it gets rid of the special meaning of
'0'.

However it needs to be documented. I attempt to do so in this PR.
Addreses bitcoin#14064.

src/init.cpp

@@ -465,7 +465,7 @@ void SetupServerArgs()
     gArgs.AddArg("-peerbloomfilters", strprintf("Support filtering of blocks and transaction with bloom filters (default: %u)", DEFAULT_PEERBLOOMFILTERS), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-permitbaremultisig", strprintf("Relay non-P2SH multisig (default: %u)", DEFAULT_PERMIT_BAREMULTISIG), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-port=<port>", strprintf("Listen for connections on <port> (default: %u, testnet: %u, regtest: %u)", defaultChainParams->GetDefaultPort(), testnetChainParams->GetDefaultPort(), regtestChainParams->GetDefaultPort()), false, OptionsCategory::CONNECTION);
-    gArgs.AddArg("-proxy=<ip:port>", "Connect through SOCKS5 proxy", false, OptionsCategory::CONNECTION);
+    gArgs.AddArg("-proxy=<ip:port>", "Connect through SOCKS5 proxy, set -noproxy to disable (default: disabled)", false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-proxyrandomize", strprintf("Randomize credentials for every proxy connection. This enables Tor stream isolation (default: %u)", DEFAULT_PROXYRANDOMIZE), false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-seednode=<ip>", "Connect to a node to retrieve peer addresses, and disconnect. This option can be specified multiple times to connect to multiple nodes.", false, OptionsCategory::CONNECTION);
     gArgs.AddArg("-timeout=<n>", strprintf("Specify connection timeout in milliseconds (minimum: 1, default: %d)", DEFAULT_CONNECT_TIMEOUT), false, OptionsCategory::CONNECTION);